1. Field of the Invention
The present invention relates generally to wet FGD scrubbers and in particular to an in-situ forced oxidation retrofit for same.
2. Description of the Related Art
Typical wet (FGD) flue gas desulfurization scrubbers consist of two major components: the scrubbing zone in which the actual gas scrubbing takes place and a recirculation tank to allow efficient utilization of the reagent. The majority of these systems are single loop systems in which the recirculation tank and the scrubbing zone are combined in one structure. The liquor sprayed in the scrubbing zone captures sulfur dioxide (SO.sub.2) forming sulfites and bisulfites. These systems run free of scale if the oxidation of sulfites to sulfates was kept below 15% or above 85%. One means of controlling scale formation in an FGD system was to force oxidize the sulfites to sulfates by bubbling air through the recirculated reagent.
Systems built more than a decade ago used to oxidize sulfites by bubbling the air through the reagent in a separate tank. The formed sulfates were separated and disposed of. These systems were referred to as ex-situ forced oxidation systems. Other systems bled a slip stream from the recirculation tank, bubbled air through the reagent slurry to oxidize the sulfites then returned it back to the recirculation tank. These systems were intermediate between the ex-situ and the more advanced in-situ oxidation systems.
Today wet scrubbers are built with the preferred means of achieving oxidation, namely performing the forced oxidation in the recirculation tank of the wet scrubber itself. This process is referred to as full in-situ oxidation and FIG. 1 depicts such a system.
In prior art FGD scrubbers, some absorbers and tanks were made of either high alloy expensive material to fight corrosion, or were made of lined carbon steel material, which is inexpensive but susceptible to corrosion and chemical attack without the use of liner materials. Liners are usually made of rubber, fiberglass, or wall paper alloys to protect the carbon steel shell from the corrosive action of the chemicals inside the tank.
Some of these prior art wet scrubbers were built and installed without the forced oxidation system being located in the recirculating tank. Retrofitting these systems to an in-situ forced oxidation system required that air be delivered and bubbled through the recirculation tank contents to achieve oxidation. Air was bubbled into the tank using sparge pipes. FIGS. 2 and 3 show such retrofits for wet scrubbers with either integral or separate recirculation tanks. These retrofits had sparge pipes penetrate the side of the tank which were supported on the opposite wall of the tank. This arrangement called for multiple wall penetrations which were particularly not welcomed in instalations using lined vessels since breaking the integrity of the liner developed areas of corrosive reagent interaction with the carbon steel under the liner. Also multiple penetrations required external space where the main air header from the compressor was located. While this arrangement is viable for new installation where equipment arrangement is not limited by existing equipment, for retrofit projects, the optimum orientation of the sparge headers usually does not fall in line with the space requirements for outside pipe routing from the compressor to the sparge pipes.
Thus it is seen that while newly built in-situ forced oxidation wet FGD scrubbers are not problem prone, the retrofit of natural oxidation wet scrubbers was fraught with problems and difficulties.